99 Classic 4.0L Automatic 240K: I purchased this Cherokee with 173K - I'm guessing that the fuel pump/filter/regulator have never been changed.

Just got back from a long road trip and for fifteen minutes one afternoon near the end, I got a slight miss at 70 mph. The miss was random and slight and felt like a momentarily clogged injector. This was in western Kansas and I was using cruise control. I noticed that it seemed to miss more consistently when going down a gentle grade (Kansas) than when going up one, but would miss on both - just more consistently when going slightly down and the CC was lifting off the accelerator. I decreased speed to 65 mph and it continued. Then I increased to speed to 75 mph and it went away. Remainder of day after a stop and next two days I drove at 70 mph with no further re-occurrence.

At the beginning of the trip I got a good miss when the CC lifted dramatically on the descent of a steep hill. Just that one occurrence on day one and nothing until two weeks later.

Fuel pump? Fuel filter/regulator? Anything else? I'm thinking it's definitely fuel related. On a bad fuel pump, wouldn't one expect it to fail more on acceleration than deceleration.

 

I'm guessing no CEL? Still could be codes stored. Might have it scanned.

 

Agree with Earl, check with a scan tool or code reader for "pending codes".

In the absence of any type of codes, yes....sure could be fuel. That can be checked very easily with a fuel pressure gauge. For a 99, you should have 49 psi at idle, plus or minus 5 psi. So never hurts to verify that. If you don't have a gauge, they can often be rented very inexpensively by your local parts stores; call around.

But could also very possibly be secondary ignition. Don't get tunnel vision on any one type of suspected problem. A miss can be caused by a LOT of things.

I'd start with fresh Champion spark plugs gapped to .035, new quality distributor cap and rotor and plug wires. Money well spent.Then see where you are at.

Unless you have (believable) evidence that the above has been done recently, all tuneup parts are always front and center for symptoms like yours. Failure to address that can result in the "chasing of your tail". Simple stuff first. Always.

Good luck and keep us updated!

 

Thanks Earl and TJ for the replies.

Correct, no CEL. And no codes.

I've put 1,500 miles on since the last and 2nd only occurrence. I'm not optimistic about discovering the underlying problem until the problem becomes more common and persistent. Accordingly, I'm going to stand down until that happens.

The purpose for the original post was to see if someone has experienced something very similar and what the ultimate resolution was.

 

Took another road trip this weekend. Problem resurfaced. I do believe the problem is that when the CC lifts dramatically it shuts off for a split second and then comes back - it doesn't completely kick off. The shut offs are repeated for a while. I can get rid of it by shutting the CC off. The shutoff activity mimics a miss. I believe now that there is no miss. I may drive it forever like this - not a big problem. That said, I'll be alert to sleuth it out. My first thought is the ECU? If not that, perhaps a sensor?

All ideas and theories welcome.